INDUCTANCE AND CAPACITY 



427 



guide wire W and must move without appreciable friction; it 

 is drawn inward by the spiral springs, F, the tension of which 

 may be regulated. When the motor is started the centrifugal 

 force causes the weight to move out in opposition to the control 

 exercised by the springs. If the speed be high enough, contact 

 between KI and K 2 will be made and the resistance short-cir- 

 cuited. The motor then slows down a little and the contact 

 is broken ; the speed then rises and the cycle is repeated. Thus 

 the speed is kept constant, subject to very slight oscillations 

 about its mean value. 



The position of the piece Q 

 to which the rear ends of the 

 springs are attached may be ad- 

 justed by the micrometer screw 

 T (pitch 1 mm.) and when prop- 

 erly adjusted may be clamped 

 in position on the rods SS. The 

 platinum contact K\, which is 

 carried by a flat spring of mode- 

 rate strength, is insulated and 

 mounted on the weight P and is 

 connected to one of the slip 

 rings by a flexible wire. The 

 contact K% is a flat plate of 

 platinum and the bar H which 

 carries it may be set at any 

 desired distance from the axis. FIG. 251. Details of Giebe speed 

 As the shaft runs through the 

 device two springs are used as indicated in the smaller figure. 



It is obvious that on account of the shaft the center of gravity 

 of the weight can never be brought to the axis of rotation. Let 

 a' be its minimum possible distance from the center of the shaft, 

 M be the mass of the weight and C the constant of the springs, 

 that is, the force in dynes necessary to extend the springs 1 cm. 

 Let r be the distance of the center of gravity of the weight from 

 the center of the shaft when the latter is rotating with an angular 

 velocity co. It will be assumed that the springs have been given 

 an initial tension Ti, that is, when the device is at rest in a 

 horizontal position and the weight is in contact with the shaft, a 



